1. Field of the Invention
The present invention relates to a flux-cored welding wire for carbon steel and a process for arc welding using the flux-cored welding wire, and more specifically, to a flux-cored welding wire used for arc welding using pure argon gas as a shielding gas and a process for arc welding of carbon steel using the flux-cored welding wire.
2. Description of the Related Art
An oxidizing gas, such as CO2, or a mixed gas of argon (Ar) and an oxidizing gas has been commonly used as a shielding gas for arc welding of carbon steel. However, in the case where welding is performed with such a shielding gas, the oxidizing gas in the shielding gas reacts with elements, such as Si and Mn, contained in a steel sheet or a wire to generate many slag components on a bead surface, disadvantageously resulting in a significant degradation of bead appearance. In traditional metal active gas (MAG) welding using CO2 as a shielding gas, a welding wire having a specific ratio (Si/Mn) of the amount of Si and the amount of Mn is reported in order to suppress the amount of slag generated (see Japanese Unexamined Patent Application Publication No. 11-320178).
In the case where slag is generated on a bead surface, when a welded joint is coated, a coating material does not adhere to only a slag portion, or the resulting coating film is peeled by the detachment of slag. As a result, while coating treatment is performed, corrosion may be promoted from a peeled portion. This problem is particularly liable to occur when a coating process, such as electrodeposition coating, which requires electrical conductivity, is performed. As a technique for improving coatability on a bead surface, for example, a metal-based-flux-cored welding wire in which graphite is added to a flux to reduce the amount of slag is reported (see Japanese Unexamined Patent Application Publication No. 2006-272405).
The use of pure Ar gas as a shielding gas without using an oxidizing gas suppresses the generation of slag. However, welding of carbon steel using pure Ar gas has been thought to be substantially impossible. The reason for this is as follows: A solid wire used for arc welding of carbon steel does not provide a thermal pinch force associated with the dissociation of an active gas in the shielding gas. A droplet is elongated and swung by, for example, an electromagnetic force. Thus, rotating transfer is liable to occur, failing to form a normal bead.
In a welding process, such as tungsten inert gas (TIG) welding, with a non-consumable electrode, it is possible to perform welding of carbon steel using pure Ar gas. In such a welding process, however, heat from electrical resistance of a wire is not generated. Compared with MAG welding or metal inert gas (MIG) welding with a wire serving as an electrode, a wire melting speed is low, so a welding speed is inevitably low, thus significantly reducing productivity. A MIG arc welding process using pure Ar shielding gas is reported (see Japanese Unexamined Patent Application Publication No. 2009-255125). In the welding process described in Japanese Unexamined Patent Application Publication No. 2009-255125, the incorporation of specific amounts of graphite and an iron powder into a flux enables welding in pure Ar gas, thereby suppressing the generation of slag and fumes.